Brain and Nervous System

What Does the Brain Do?

The brain controls what you think and feel, how you learn and remember, and the
way you move and talk. But it also controls things you're less aware of — like the
beating of your heart and the digestion of your food.

Think of the brain as a central computer that controls all the body's functions.
The rest of the nervous system is like a network that relays messages back and forth
from the brain to different parts of the body. It does this via the spinal
cord, which runs from the brain down through the back. It contains threadlike
nerves that branch out to every organ and body part.

When a message comes into the brain from anywhere in the body, the brain tells
the body how to react. For example, if you touch a hot stove, the nerves in your skin
shoot a message of pain to your brain. The brain then sends a message back telling
the muscles in your hand to pull away. Luckily, this neurological relay race happens
in an instant.

What Are the Parts of the Nervous System?

The nervous system is made up of the central nervous system and the peripheral
nervous system:

The brain and the spinal cord are the central nervous system.

The nerves that go through the whole body make up the peripheral nervous
system.

The human brain is incredibly compact, weighing just 3 pounds. It has many folds
and grooves, though. These give it the added surface area needed for storing the body's
important information.

The spinal cord is a long bundle of nerve tissue about 18 inches long and 1/2-inch
thick. It extends from the lower part of the brain down through spine. Along the way,
nerves branch out to the entire body.

The brain and the spinal cord are protected by bone: the brain by the bones of
the skull, and the spinal cord by a set of ring-shaped bones called vertebrae. They're
both cushioned by layers of membranes called meninges and a special fluid called cerebrospinal
fluid. This fluid helps protect the nerve tissue, keep it healthy, and remove waste
products.

What Are the Parts of the Brain?

The brain has three main sections: the forebrain, the midbrain, and the hindbrain.

The Forebrain

The forebrain is the largest and most complex part of the brain. It consists of
the cerebrum — the area with all the folds and grooves typically seen in pictures
of the brain — as well as other structures under it.

The cerebrum contains the information that essentially makes you
who you are: your intelligence, memory, personality, emotion, speech, and ability
to feel and move. Specific areas of the cerebrum are in charge of processing these
different types of information. These are called lobes, and there are four of them:
the frontal, parietal, temporal, and occipital lobes.

The cerebrum has right and left halves, called hemispheres. They're connected in
the middle by a band of nerve fibers (the corpus callosum) that lets them communicate.
These halves may look like mirror images of each other, but many scientists believe
they have different functions:

The left side is considered the logical, analytical, objective side.

The right side is thought to be more intuitive, creative, and subjective.

So when you're balancing your checkbook, you're using the left side. When you're
listening to music, you're using the right side. It's believed that some people are
more "right-brained" or "left-brained" while others are more "whole-brained,"
meaning they use both halves of their brain to the same degree.

The outer layer of the cerebrum is called the cortex (also known
as "gray matter"). Information collected by the five senses comes into the
brain to the cortex. This information is then directed to other parts of the nervous
system for further processing. For example, when you touch the hot stove, not only
does a message go out to move your hand, but one also goes to another part of the
brain to help you remember not to do that again.

In the inner part of the forebrain sits the thalamus, hypothalamus, and
pituitary gland
:

The thalamus carries messages from the sensory organs like the
eyes, ears, nose, and fingers to the cortex.

The hypothalamus also controls the pituitary gland, which makes
the
hormones
that control growth, metabolism, water and mineral balance, sexual maturity,
and response to stress.

The Midbrain

The midbrain, underneath the middle of the forebrain, acts as a master coordinator
for all the messages going in and out of the brain to the spinal cord.

The Hindbrain

The hindbrain sits underneath the back end of the cerebrum. It consists of the
cerebellum, pons, and medulla. The cerebellum — also called the "little
brain" because it looks like a small version of the cerebrum — is responsible
for balance, movement, and coordination.

The pons and the medulla, along with the midbrain, are often called the brainstem.
The brainstem takes in, sends out, and coordinates the brain's messages. It also controls
many of the body's automatic functions, like breathing, heart rate, blood pressure,
swallowing, digestion, and blinking.

How Does the Nervous System Work?

The basic workings of the nervous system depend a lot on tiny cells called neurons.
The brain has billions of them, and they have many specialized jobs. For example,
sensory neurons send information from the eyes, ears, nose, tongue, and skin to the
brain. Motor neurons carry messages away from the brain to the rest of the body.

All neurons relay information to each other through a complex electrochemical process,
making connections that affect the way you think, learn, move, and behave.

Intelligence, learning, and memory. As you grow and learn,
messages travel from one neuron to another over and over, creating connections, or
pathways, in the brain. It's why driving takes so much concentration when someone
first learns it, but later is second nature: The pathway became established.

In young children, the brain is highly adaptable. In fact, when one part of a young
child's brain is injured, another part often can learn to take over some of the lost
function. But as you age, the brain has to work harder to make new neural pathways,
making it harder to master new tasks or change set behavior patterns. That's why many
scientists believe it's important to keep challenging the brain to learn new things
and make new connections — it helps keeps the brain active over the course of a lifetime.

Memory is another complex function of the brain. The things you've done, learned,
and seen are first processed in the cortex. Then, if you sense that this information
is important enough to remember permanently, it's passed inward to other regions of
the brain (such as the hippocampus and amygdala) for long-term storage and retrieval.
As these messages travel through the brain, they too create pathways that serve as
the basis of memory.

Movement. Different parts of the cerebrum move different body
parts. The left side of the brain controls the movements of the right side of the
body, and the right side of the brain controls the movements of the left side of the
body. When you press your car's accelerator with your right foot, for example, it's
the left side of your brain that sends the message allowing you to do it.

Basic body functions. A part of the peripheral nervous system
called the autonomic nervous system controls many of the body processes
you almost never need to think about, like breathing, digestion, sweating, and shivering.
The autonomic nervous system has two parts: the sympathetic nervous system and the
parasympathetic nervous system.

The sympathetic nervous system prepares the body for sudden stress,
like if you witness a robbery. When something frightening happens, the sympathetic
nervous system makes the heart beat faster so that it sends blood quickly to the different
body parts that might need it. It also causes the
adrenal glands
at the top of the kidneys to release adrenaline, a hormone that helps
give extra power to the muscles for a quick getaway. This process is known as the
body's "fight or flight" response.

The parasympathetic nervous system does the opposite: It prepares
the body for rest. It also helps the digestive
tract move along so our bodies can efficiently take in nutrients from the food
we eat.

The Senses

Sight. Sight probably tells us more about the world than any other
sense. Light entering the eye
forms an upside-down image on the retina. The retina transforms the light into nerve
signals for the brain. The brain then turns the image right-side up and tells you
what you're seeing.

Hearing. Every sound you hear is the result of sound waves entering
your ears and making your eardrums
vibrate. These vibrations then move along the tiny bones of the middle ear and turn
into nerve signals. The cortex then processes these signals, telling you what you're
hearing.

Taste. The tongue contains small groups of sensory cells called
taste buds that react to chemicals in foods. Taste buds react to sweet, sour, salty,
bitter, and savory. The taste buds send messages to the areas in the cortex responsible
for processing taste.

Smell. Olfactory cells in the mucous membranes lining each nostril
react to chemicals you breathe in and send messages along specific nerves to the brain.

Touch. The skin
contains millions of sensory receptors that gather information related to touch, pressure,
temperature, and pain and send it to the brain for processing and reaction.